Fuel cells and semi fuel cells that use hydrogen peroxide or oxygen as oxidant are environmentally friendly methods for generating electricity. They don't produce toxic reaction products during discharge as water is the only product of the use of these oxidants in fuel cells according to the reactionsH2O2+2 H3O++2 e−→4 H2O, respectively,   (1)O2+4 H3O++4 e−→6 H2O.   (2)The electrochemical potential calculated from thermodynamic data is +1.77 V for reaction (1) at pH=0, +1.23 V for reaction (2) but the values reached in practice are significantly lower especially for prior art fuel cell cathodes at current densities of 40-100 mA/cm2.
High performance fuel cells would be a perfect power supply for electrically powered cars as fuel cells can reach an efficiency that is much larger than the efficiency of combustion engines which is limited by the Carnot efficiency η=1−T1/T2 determined by the temperatures T1 of the cold and T2 of the hot reservoir and reaches values of typically less than 40% while a fuel cell could reach higher efficiencies. Moreover fuel cells are intrinsically safer than lithium batteries of the same capacity as only small amounts of educts are present in the fuel cell at the same time and can be stored separately while all highly reactive lithium metal and cathode material, an oxidizer, are mounted next to each other so damage of the separator may result in a violent exothermic reaction of the whole lithium stored in the battery.
However reaction (1) is rather slow and requires more efficient electrocatalysts in order to reach a low polarization at current densities of 10 mA/cm2 and above. Cathodes with prior art electrocatalysts still cannot reach low polarizations at current densities of 100 mA/cm2 and above and suffer from other disadvantages like strong hydrogen peroxide decomposition.
In spite of substantial efforts to develop improved oxygen and hydrogen peroxide cathodes for fuel cells during the last five decades the power density that could be reached by such fuel cells is still fairly limited as the polarization of the cathodes is already quite large at rather small current densities due to small value of the exchange current densities j0 for the above reactions (1) and especially (2).
In addition rather large amounts of very expensive catalysts like platinum and platinum alloys have to be used in order to reach current densities required for an electrically powered car as the catalyst utilization is quite low (about 9% for typical PEM-fuel cells). An estimate for the manufacturing costs of the electrodes of a fuel cell for an electrically powered car was $50-100 per kW according to S. Srinivasan (“Fuel Cells”, Springer, 2006, p. 603). For an electrically powered car with the performance of conventional cars (80 kW power) manufacturing costs of $4000-$8000 for the electrodes alone would be therefore expected.
State of the art fuel cell electrodes for polymer electrolyte membrane (PEM) fuel cells are produced by a coating process using an ink of catalyst mixed with a dispersion of fluoropolymer ionomer like copolymers of tetrafluoroethylene and perfluorovinylether sulfonic acid commonly sold under the trademark “NAFION” by E.I. DuPont de Nemours and Company, Wilmington, Del. Such an electrode is shown in FIG. 1. The supported catalyst (104) with platinum, palladium or iridium electrocatalyst centers (106) is randomly distributed in the catalyst-ionomer layer (102) formed from the ink on a conducting current collector (100). Up to now in spite of tremendous research efforts over many decades researchers didn't recognize the disadvantages that arise from this random electrode structure.
“NAFION” is an ion conductor that is not electron-conducting. But in order to act as an electrocatalyst a catalyst particle must take up electrons from the current collector of the cathode as it is shown by the arrows in FIG. 1 illustrating the flow of electrons within the cathode. Therefore catalyst utilization is reduced by the random dispersion of the catalyst in a non-electron-conducting polymer as only the fraction of the catalyst that is in electrical contact to the current collector is acting as electrocatalyst for production of electrical energy.
Moreover prior art catalysts like platinum, palladium-iridium or gold for hydrogen peroxide cathodes according to reaction (1) show strong polarization at rather small current densities of 10 mA/cm2. According to the literature magnesium/hydrogen peroxide-semi fuel cells (open circuit voltage 2.1 V) with conventional cathodes can deliver only a voltage of 1.3 V at current densities of 40 mA/cm2 and 25 ml/min flow rate. The situation is similar for oxygen cathodes according to reaction (2) due to the very low exchange current density j0.
Besides efficient prior art electrocatalysts like palladium-iridium (50 atomic-%) cannot be used in concentrated catholyte solutions comprising hydrogen peroxide (c(H2O2)>0.5 mole/l) that would be useful for high power density fuel cells that operate at high current densities because of decreasing efficiency of prior art electrocatalyst palladium-iridium (50 at.-% Ir) at c(H2O2)>0.25 mole/l for reaction (1). This prior art electrocatalyst generates much oxygen by catalytical hydrogen peroxide decomposition according to (3) 2 H2O2→2 H2O+O2. The energy density decreases from over 700 Wh/kg (for c(H2O2)=0.03 mole/l) to about 400 Wh/kg (for c(H2O2)=0.25 mole/l) because of this parasitic reaction instead of an expected increase due to the reduced mass of the catholyte because of the reduced water content in the catholyte as a result of the increased hydrogen peroxide concentration.
Information relevant to attempts to address these problems can be found in U.S. patent applications No. 2008/0182153 A1, 2008/0193827 A1, 2008/0063922 A1, 2008/0054226 A1, 2004/0224218 A1, 2004/0191605, U.S. Pat. Nos. 7,175,930, 5,296,429, 5,445,905, 6,465,124 and the articles Electrochemistry Communications 10 (2008), 1610, in print, Journal of Power Sources 165 (2007), 509 and Journal of Power Sources 164 (2007), 441.
However, each one of these references suffers from one or more of the following disadvantages as long diffusion paths for educts (the oxidants H2O2 or O2 and H3O+) and products (H2O) from the electrolyte to the electrocatalyst and vice versa, limited durability of electrodes, high costs of the catalysts, high manufacturing costs due to complicated manufacturing processes, strong decomposition of hydrogen peroxide at the surface of the catalyst and low utilization efficiency of hydrogen peroxide, impracticality of the use of concentrated solutions of hydrogen peroxide, strong polarization at large current densities and low utilization of the catalyst due to a missing conduction path for electrons.
For the foregoing reasons, there is a need for hydrogen peroxide cathodes and oxygen cathodes for fuel cells that are more efficient, less expensive to manufacture and durable and that can deliver higher current densities with lower polarizations and that can be operated in concentrated solutions of hydrogen peroxide.